Variable pitch propellers



Aug- 4, 1964 R. T. ELMES ET AL 3,143,175

VARIABLE PITCH PROPELLERS Filed Oct. 13, 1960 3 Sheets-Sheet 1 /26 fI) o rF. RT /-EBEn/e fi E L M E. s

M AfwHun Harmful LAME Aug. 4, 1964 R. T. ELMr-:s ETAL VARIBLE PITCH PRoPELLERs .'5 Sheets-Shea?l 2 Filed Oct. 13, 1960 Aug. 4, 1964 R. T. ELMEs ErAl.

VARIABLE PITCH PROPELLERS 3 Sheets-Sheet 5 Filed Oct. 13, 1960 United States Patent O 3,143,175 VARABLE PITCH PRPELLERS Robert Terence Elmas and Arthur Hampton Lane, Cheltenham, England, assignors to Dowty Rotol Limited, Cheltenham, England, a British company Filed Oct. 13, 196i), Ser. No. 62,485 Claims priority, application Great Britain st. 16, i959 6 Claims. (Cl. 17d-160.32)

This invention relates to variable pitch propellers.

According to the present invention there is provided a variable pitch propeller having pitch control means cornprsing a hydraulic piston-and-cylinder motor connected to the propeller blades to alter the pitch thereof, a plurality of stops operable to prevent relative movement of the piston and cylinder, in a sense to fine the propeller pitch, beyond respective predetermined fine pitch settings, supply conduits for supplying pressure lluid to opposite ends of the cylinder of said motor, a source of pressure fluid, a further fluid conduit which is independent of said supply conduits and which is connected to said source, means for effecting successive increases in the pressure in said further fluid conduit, and withdrawal means for withdrawing said stops successively when said successive increases of pressure occur in said further fluid conduit.

One embodiment of the invention will now be described by way of example with reference to the accompanying diagrammatic sectional drawings in which the arrangement is shown broken into FIGURES 1A, 1B and 1C for convenience.

FIGURE 2 illustrates a modification, and

FIGURE 3 illustrates a second modification.

Referring to the drawings, the propeller is generally indicated by the reference numeral 11 and comprises a hub 12 which carries propeller blades 13 which are adjustable as to pitch about their longitudinal pitch change axes 14. Each blade comprises a crank pin 15 at its root end coupled by a connecting rod 16 to the ram 17 of a pitch change motor. The rain 17 is slidable in a cylinder 18 to adjust the pitch of the propeller. The cylinder 18 is arranged coaXially with the driving shaft 19 of the propeller and in the nose portion of the hub 12. The pitch adjustment is controlled by a constant speed unit generally indicated by the reference numeral 20. This unit is of well-known construction and comprises a governor 21 and valve 22. Associated with the governor 21 is a pump 23 which receives pressure fluid from the oil system of the associated engine through a conduit 24, or alternatively draws it from a propeller sump fixed to the engine or forming part of the propeller hub structure.

The pump 23 delivers pressure fluid to the valve 22 which in turn controls the passage of the pressure fluid from the pump to the propeller pitch change motor by way of either conduit 25 or 26 depending upon the nature of the pitch change, that is whether the blades are moved to a coarser or finer pitch respectively. The conduits 25, 26 communicate with ducts 27 and 28 passing through the hub 12 and leading to opposite sides of the rarn 17.

When the pressure iluid from the pump 23 is delivered by conduit 26 and duct 28, the ram 17 is moved forwardly to bring the blades into finer pitch position, and when delivered by conduit 25 and duct 27, the ram is moved rearwardly to bring the blades into a coarser pitch position.

The ram 17 is of annular form and slides on a stationary barrel 29 housing first, second and third stop means operative to prevent movement of the ram 17 relative to the cylinder 18 forwardly, that is to the left in the drawing, to adjust the propeller into a pitch less than a first predetermined pitch, a second predetermined pitch which is less than said first predetermined pitch and a third predetermined pitch which is less than said second 3,l43,l Patented Aug. 4., 1964 moving inwardly by a stop retaining member in the form of a ring 32 which is slidably mounted upon a core 33 within the barrel 29. The ring 32 has a stepped outer periphery having a radially outer step 32b, a step 32C disposed radially inwardly of the step 32b. A radially inner step 32d, and a coil spring 34 is interposed between the ring and an inwardly directed flange 35 formed on the inner wall of the barrel 29.

The stops 30 may lie in the path of movement of one or more of the shoulders 36a, 36!) and 36C formed on the inner diameter of a stepped sleeve 36 projecting forwardly from and forming part of the ram 17 in dependence upon the position of ring 32 as hereinafter described. These shoulders normally limit the movement of the ram to the left in the drawing depending upon the position of the stops 30 as hereinafter described. The barrel 29 is provided with suitable slots 29a through which the stops 30 project radially outwardly, and extends forwardly beyond the stops. An external radial flange 37 engages with and is connected to a flange 38 integral with the forward end closure member 69 of the cylinder 18.

The ring 32 is provided at its forward end with an inturned part or flange 32e which when the ring 32 is in its extreme rearward position engages a forwardly facing shoulder 39a formed on a sleeve 39. The sleeve 39 has an annular jack piston 40 formed integrally therewith at its forward end, and is slidable upon a reduced diameter part 33a of the core 33. The piston 40 works in a cylinder formed by the forward part of the barrel 29, the flange 35 and the forward end closure member 69 of the cylinder 18.

Slidably mounted on a part 33C of the core 33 to the rear of the ring 32 is a sleeve 42 having a pair of radially inwardly directed flanges forming annular jack pistons 43 and 44, these pistons working respectively in annular peripheral recesses or cylinders 45 and 46 in the core 33. A coil spring 46a is provided in the cylinder 46 urging the piston 44 rearwardly and the sleeve 42 extends forwardly into abutment with ring 32.

The core 33 has a central bore 47 which houses a piston valve 48 slidable therein. The bore is stepped to provide an increased diameter part. A flange 48a on the piston valve is arranged to abut the step in the bore to limit movement of the valve 48 in the rearward direction. A coil spring 49 is interposed between the flange 48a and the forward end of the bore 47. The outer diameter of the piston valve 48 is provided with an annular recess 50 which, when the piston valve is in the rearward position (as shown in the drawing), places a passageway 51 in communication with a drain duct 52. Passageway 51 is piston 44. Cylinders 45 and 46 are in communication with the drain passageway 52 at their forward ends.

The piston valve 48 is provided with a further annular recess 53 spaced forwardly from the annular recess 50 and in communication with the hollow interior of the piston valve 48 through ports 55.

The chamber 56 to the rear of the piston valve 48 is in communication through a passageway 57 with the fine pitch duct 28. The duct 28 also leads hydraulic fluid under pressure to the rearward, that is to say, the fine pitch side of the ram 17 through a branch duct 58. The bore 47 forward of the piston valve 48 communicates with the coarse pitch duct 27 through a duct 59. The duct 27 leads hydraulic fluid under pressure to the forward, that is to say, the coarse pitch side of the ram i7 and also through the branch duct 59 to the forward end of bore 47, the hollow interior of the piston valve 118 and through the ports 55 to the annular recess 53.

A duct 69 or third oil line passes through the hub in convenient manner aud communicates at its forward end with the rearward end of a bore 61 disposed coaxially with and just forward of the bore 47. The bore 61 houses a valve spool 62 having three spaced lands 63, 64 and 65. The spool 62 is urged rearwardly by a rst coil spring 66 interposed between land 63 and a vented stop member 67. The member 67 is itself urged rearwardly by a second coil spring 68 interposed between the forward face of the member and a suitable abutment forming part of the closure member 69 of the cylinder 18. Rearward movement of the member 67 is limited by the provision of a step in the bore 61.

Considering the spool 62 to be in its rearmost position (as shown) the land 65 closes over the end of a passageway 71 which, when the jack piston 40 is in its rearmost position also, opens at its other end into an annular chamber 72 formed on the inner diameter of the sleeve 39. A port 73 places the chamber 72 in communication with the chamber 74 formed on the rearward side of the jack piston 4i). With the spool 62 in this position a branch passageway 71a places the annulus '75 formed between lands 6d and 65 in communication with the passageway 71. Also a passageway 76 which opens into the annular chamber 72 a predetermined distance forward of the passageway 71 is in communication with the annulus 77 formed between lands 63 and 64 of the spool 62. A drain passageway 78 at all times places the annulus 77 in communication with cylinder 45 and thus with the drain duct 52.

A passageway 79 branches from the forward end of the third oil line duct 6@ and leads to an annular chamber Sii formed in the outer surface of the core 33 and bounded by the ring 32. The inner diameter of the ring is provided with an annular recess 81 which communicates with a drain channel S2, itself connecting through cylinder 45 to drain duct 52, to provide a means of lock indication in a manner fully disclosed in U.S. Patent No. 2,934,154.

The part of the bore 61 forward of the spool 62 is in communication with the chamber 83 on the forward side of the jack piston 40 through a passageway 84, while chamber 83 is in communication through ports 85 and 86 with the coarse pitch side of the ram 17.

The duct 6i) communicates with a'third oil line conduit 37 which communicates with the delivery side of the provided valve 9@ also is open and valve 95 open (or alternatively by-pass valve unit 97 has been moved to a position in which valve 95 is by-passed and fluid delivery is direct from conduit 96 through conduits 98 and 102 to valve 9d) then pressure fluid will pass into a conduit 197 which leads to the underside of piston 104 thereby to displace the piston and increase the loading in the spring of the third oil line relief valve 103. Hence the operating pressure in the third oil line 87 may be increased according to the new setting of the spring. At the same time pressure fluid passes from conduit 107 into a conduit 16S branched therefrom which communicates with a cylinder 109 formed at the upper end in the drawing of the casing 110'of the constant speed governor 21.

Slidably mounted within the cylinder 109 is a piston 111 having a push rod 112 formed therewith. Under the hydraulic pressure acting upon it the piston 111 is displaced downwardly in the cylinder 109 and the push rod overrides Y the effect of the flyweights 113 of the governor and dispump 23 through a valve device 88 provided in a conduit Y 89. The valve device 38 is opened to permit dow of hydraulic fluid into the third oil line only when a solenoid operated Valve 90 is Opened to transmit pressure luid through a conduit 91 to displace a piston 92 connected with the valve member 93 of the device S8 against the eifort of a coil spring 94;

In the system being described there are three other solenoid operated valves 95, 1&5 and 114 similar to valve 9i?. Of these, valve 95 receives pressure fluid through a conduit 96 taken from the delivery side of the pump 23 and passing through a by-pass valve unit 97. When the valve 95 is open pressure iiuid passes through the unit 97 and conduit 9S to the underside of a piston 99 ina cylinderiiii thereby displacing the piston to gag the relief valve 101 of the fine pitch conduit 26 and thereby afford means for increasing the operating pressure in the fine pitch circuit. At this time the pressure fluid in conduit 98 is transmitted to the solenoid operated valve 9) through a conduit 192. As soon as valve 90 is opened pressure fluid passes through conduit 91 to open the valve device 88 and permit pressure fluid from conduit 89 to enter the third oil line conduit 87.

Selection of third oil line relief valve gagging is achieved by opening of a further solenoid operated valve 1%5 which receives pressure fluid only when valve 90 is open since the supply conduit 106 is taken from the outlet side of valve 90 to valve 105. When valve 195 is opened, then places the valve 22 downwardly such that the ne'pitch pressure conduitV 26 is fully opened to the delivery of pressure fluid from pump 23-and the coarse pitch pressure conduit 25 is opened to drain.

The fourth solenoid operated valve 114 is provided in the present system to provide forunreversing and also the automatic pitch coarsening function as fully described in our British Patent No. 790,226. T his valve receives its pressure fluid supply from conduit 96 and when opened directs Huid through a conduit 115 to the underside of a piston 116 also in the governor casing 110. Consequent upward movement of this piston lifts the governor valve 22 such that fullcoarsening pressure is applied through conduit 25.

Thus in the system now described the solenoid operated valve 95 in providing for increased tine pitch pressure is capable of initiating withdrawal of the cruising pitch stop, the valve in providing for operation of the third oil line at a first pressure level is capable of initiating withdrawal of the Hight Vfine pitch stop, the valve 105 in providing for operation of the third oil line at a second higher pressure level is capable of initiating withdrawal of a superne pitch stop and the valve 114 is capable of initiating automatic coarsening of the propeller blades.

Each of the solenoid operated valves 95, 90, 105 and 114 has drain connections which connect with a common drain conduit 117. This conduit is provided with a nonreturnvalve 11S which maintains the pressure in the conduit at 5 lbs. per square inch.

When it is required to operate the first stop withdrawal means in order to render the first stop means inoperative, the solenoid of the valve is energized to open the valve. This may be achieved by means of a control system as described in either co-pending US. application SerialNo. 697,07() or U.S. Patent No. 2,994,385 in the case where the propeller is one of a number of propellers installed in an aircraft having a plurality of gas turbine engine driven propellers.

When it is required to operate the second stop withdrawal means in order to render the second stop means inoperative the solenoid of the valve 90 is energized to open the valve. This may be achieved only after touch down of the aircraft and is controlled by a-master lever operable by the pilot. This lever is not free to move to its operative position until the throttle lever of the engine driving the propeller has been moved to a predetermined low throttle setting thus ensuring that the flight fine pitch stop cannot be withdrawn when the propeller is rotating above a predetermined r.p.m. Such a systemis disclosed in U.S. application Serial No. 687,204.

Alternatively, however, the solenoid of` the valve 90 may not be energized until an oleo switch associated with the aircraft undercarriage has closed consequent upon touch down of the aircraft.

When it is required to operate the withdrawal means in order to render the third stop means inoperative vthe solenoid of the valve is energized to open the valve.

As with the second stop, this may be achieved only after touch down of the aircraft and is controlled by a further range of movement of the master lever beyond that for removal of the second stop.

Alternatively if the second stop is removable automatically on the closure of an oleo switch, then the master lever may only be operative for selecting withdrawal of the third stop.

The operation of the mechanism contained within the propeller hub 12 under the control of the solenoid operated valves 90, 95 and 105 will now be described.

During normal flight the pitch change range of the propeller is limited in the pitch fining direction at a Hight cruise position by the stops 30 which are held out in the path of the shoulders 36a, 36h and 36C by the step 32b of the ring 32, this step then engaging with the ring of stops 30 and the shoulder 36a then cooperating with the stops 30 to form a night cruise stop when the shoulder 36a comes up against rearward chamfers on the stops 30. Under these conditions the ring 32 is in its extreme rearward position and is held in this position by the coil spring 34. The flight cruise stop in the example being described corresponds to a 32 degree pitch angle of the propeller blades, and is provided to prevent undue windmilling speeds of the propeller occurring in the event of a failure of the constant speed unit controlling the pitch of the propeller allowing the propeller to fine off while the aircraft is moving at a high forward speed.

When it is required to adjust the propeller into a pitch less than 32 degrees, for example when preparing to land, the propeller is fined olf on to the Hight cruise stop. This is usually achieved by setting the datum of the constant speed unit controlling the pitch of the propeller so that the constant speed unit calls for a propeller pitch which is less than that set by the flight cruise stop. The fine pitch pressure relief valve is then gagged by energizing the solenoid of the valve unit 95, as already explained. Thus when the shoulder 36a comes up against the stops 30 and the ram 17 is prevented from further movement in the pitch fining direction, the pressure on the fine pitch side of the hydraulic system of the pitch change motor is able to rise above the normal operating maximum value, the increased pressure transmitted to the chamber 56 overcoming the action of the spring 49 and displacing the piston valve 48 forwardly so as to open the passageway 51 to chamber 56. Fluid under pressure is then delivered from the fine pitch conduit 26 and duct 28, passageway 57, chamber 56 and passageway 51 to chambers 45b and 4611, and the jack means comprising the pistons 43 and 44 is operated so that the sleeve 42 is displaced forwardly thereby pushing the ring 32 forwardly to bring the step 32e on the ring radially within the ring of stops 30. Operation of the jack means as just described is accompanied by the exhaust of hydraulic fluid from the chambers 45C and 46c through the drain duct 52.

The resilient lingers 31 constitute spring means which tend to urge the stops 30 into a position in which they lie in the path of movement of the shoulders 3611 and the shoulder 36C but out of the path of movement of the shoulder 36a.

The sleeve 42 has a limited range of forward movement suicient to displace the ring 32 from a first position in which the step 32b lies radially within the ring of stops 30 through a rst range of movement to bring the ring to a position in which the stops spring onto the diameter 32C of the ring.

As soon as the ring 32 has been displaced as just described the ram 17 becomes free to slide over the stops 3f) until the shoulder 36h on the ram comes up against the stops.

lf, when the propeller has been ned olf on to the ight cruise stop and the ring 32 has been displaced to its position as shown in the drawing, the constant speed unit is still demanding a finer propeller pitch, the ram 17 will immediately move forward over the stops 30 in the manner described, thus relieving the pressure in the line pitch conduit 2S and duct 26 and therefore in chamber 56. The valve 4S will consequently move back under the action of the spring 49 to its position as shown in the drawing. In these circumstances, however, the spring 34 is unable to return the ring 32 to its rearmost position since the stops 30 are prevented from moving outwardly by the sleeve 36 which is now enclosing the stops at its forward end.

The shoulder 36h cooperates with the stops 30 to form a flight fine pitch stop which, in the present example is set at a propeller pitch of 20 degrees. The flight ne pitch stop is operative to prevent pitch ning movement of the propeller to a pitch less than 20 degrees during all phases of ight, and is required to be removed, that is to say rendered inoperative, only after touch down of the aircraft in order that the propeller may be moved into a finer pitch range for the purposes of braking the aircraft on the runway by creating a desired degree of propeller drag.

In order to allow the propeller to fine off to a pitch less than 2O degrees, hydraulic fluid under pressure is permitted to enter the third oil line 87 and 60 when the solenoid of the valve is energized and consequently the valve 93 is opened as previously explained. Fluid pressure is thus conveyed through duct 60 into the bore 61 at its rearward end to act on the rearwardly facing annular area of the land 65 of the valve spool 62. In consequence the spool 62 is displaced forwardly in the bore 61 against the effort of the coil spring 66 so that the land 65 opens the end of the passageway 71 at the bore 61 to pressure fluid. At the same time the land 64 closes off the end of the passageway 76 at the bore 61, thereby closing passageway 76 from its connection with drain passageway 78. Thus pressure Huid is transmitted through chamber 72 and ports 73 to the chamber 74a, whereupon the jack piston 40 is operated and the sleeve 39 moves forwardly on the core 33 and the shoulder 39a engages behind the flange 32e whereafter the continued forward movement of the sleeve 39 displaces the ring 32 further forward through a second range of movement against the action of the spring 34 to bring the step 32d of the ring radially within the ring of stops 30 so that the shoulder 36b is free to force the stops inwardly on to the step 32d thereby freeing the ram 17 for further forward sliding movement in the cylinder 18, and over the stops, to adjust the propeller into a still finer pitch range limited ultimately by the shoulder 36o on the ram when this shoulder comes up `against the stops 30 this constituting a superfne pitch stop.

Following a predetermined forward displacement of the jack piston 40 the sleeve 39 closes olf the end of passageway 71, whereupon the fluid pressure applied in chamber 74 is cut olf. Thus the piston 40 is hydraulically held in its new position. If leakage of oil from the chamber 74 is suiiciently great to cause the piston 40 to move rearwardly before such time as they sleeve 36 of the ram 17 moves further forwardly over the stops 30, then the end of the passageway 71 will be cracked open to make up the losses and restore the piston 40 to its new position.

Propeller blade switches are provided such that as the ram moves into this still hner pitch range the solenoids of the valves 90 and 95 are de-energized and thus the valve 93 is closed and the third oil line is communicated with drain. As soon as this occurs the spool 62 moves rearwardly in the bore 61 under the effort of coil spring 66 thereby communicating chamber 74 with drain through port 73, chamber 72, passageway 76, annulus 77, passageway 78, chamber 45 and duct 52. Thus the spring 34 will tend to push the ring 32 back to its rearmost position. This is however prevented by the ram which holds the stops 3 0 inwardly on the step 32d of the ring. As soon `as the pitch of the propeller is coarsened to an angle greater than 20 degrees, however, the spring 34 becomes free to push the ring 32 rearwardly to bring the stop onto algarve the step 32C of the ring 32 thereby resetting the flight tine pitch stop, whilst coarsening movement to an angle greater than 32 degrees allows the night cruise stop to be reset, the spring 34 pushing the ring 32 to its rearmost position together with the sleeve 42, and the ring 32 forcing the stops 30 outwardly into the path of movement of the shoulder 36a.

From the foregoing description it will be seen that the jack means comprising the pistons 43 and 44 is operable independently of movement of the jack piston 4t), and vice versa. It will also be seen that the jack piston 40 is 0perable by hydraulic fluid under pressure supplied through the third oil line to displace the ring 32 from its rearmost or lirst position through both its first and second ranges of movement if desired. The jack means comprising the piston 40 may therefore be actuated to render both the flight cruise stop and the flight tine pitch stop inoperative in one operation.

As previously explained, the shoulder 36C cooperates with the stops 30 to form a superfine pitch stop which in the present example is set at a propeller pitch of zero degrees. The superne pitch stop is operative to prevent pitch flning movement of the propeller below the zero degree position into the reverse pitch range and is required to be removed, that is to say rendered inoperative only when the aircraft is on the ground for the purpose of braking the aircraft on the runway by reversing the propeller pitch.

In order to allow the propeller to move below the zero degree position the three solenoids respectively of the valves 95, 90 and 105 are all energised, the first (95) to effect hydraulic supply to the valve 90, the second (90) to open the valve 93 to permit pressure uid to pass into the third oil line, and the third (105) to effect displacement of the piston 104 to gag the relief valve 103, as previously described.

l The pressure in the third oil line is thus conveyed through duct 60 into the bore 61 at its rearward end to act on the rearwardly facing annular area of the land 65 of the valve spool 62. In consequence the spool 62 is displaced forwardly in the bore 61 against the effect of the coil spring 66 so that the land 65 opens the end of the passageway 71 at the bore 61 to pressure fluid. The spool 62 moves until it abuts the vented stop member 67 where- -upon as previously explained the piston 40 moves forwardly until the end of the passageway "l1 opening into the chamber 72 is closed off by the step at the rearward end of the annular chamber 72. Thus the fluid pressure supply to the chamber 74 is cut-off and the pressure in passageway 71, duct 69 and conduit 87 builds up. The extent of build up is limited only by the new gagged blow-off setting of the relief valve 103, this being of the order of 600 p.s.i. Hence with the now increased operating pressure communicated to the rearward end of the bore 61, the valve spool 62, spring 66 and stop member 67, are displaced bodily forwardly against the effort of the coil spring 68 until the land 64 fully opens that end of the passageway 76 which enters the bore 61. Thus the higher pressure fiuid already communicated through passageway 71a to annulus 75 is now in communication through passageway 76 and chamber 72 with the chamber 74a so that a rapid displacement of the jack piston 40 in the forward direction now occurs.

This forward movement of the piston 40 and of the sleeve 39 displaces the ring 32 further forwardly against the action of the spring 34 to bring the step 32d forward and clear of the stops 30 so that the shoulder 36C is free to force the stops inwardly into the space immediately rearward of the ring 32, thereby freeing the ram 17 for further forward sliding movement in the cylinder 18, and over the stops, to adjust the propeller into the reverse pitch range, where in the example being described at an equivalent ram position of -15 degrees, the ram abutment 17a comes up against the shoulder stop 18a formed in the cylinder 18,

Propeller blade switches are provided such thatas the ram moves into reverse pitch the solenoids of the valves 9), 95 and 105 are rie-energized and thus the valve 93 is closed and the relief valve 163 is ungagged, the third oil line then being placed in communication with drain.

As soon as this occurs the spool 62 moves rearwardly in the bore 61 under the effort of coil springs 66 and 68 thereby communicating chamber 74 with drain through port 73, chamber 72, passageway 76, annulus 77, passageway 78, chamber 45C and duct 52. Thus the spring 34 will tend to push the ring 32 back to its rearmost position. This is however prevented by the ram which holds the stops 30 fully inwardly into the space rearward of the ring 32.

By the provision of three stops at 32 degrees, 20 degrees and zero degrees the pilot is able to select aerodynamic braking by the blades at zero degrees where a relatively long landing run is not objectionable or alternatively may select full reverse pitch braking at -15 degrees where a relatively short landing run is desirable. Also the zero degree stop provides a suitable low pitch position for starting of the engine, this being of particular importance in propeller turbine engines in which the propeller system is directly coupled with the compressor system.

The system hereinbefore described by way of example has referred to the provision of means for increasing the third oil line pressure once only. It may however be very desirable to increase this pressure say three times to give four levels of pressure operation and the system so arranged to ,remove a similar number of stops. Thus the system is basically a step by step follow up system.

As in the arrangement described above, the jack piston 4d may be provided with a stepped movement simply by a valve coaxially arranged in co-operation with it. In this case for each position of the valve the piston would have a unique follow-up position, the valve itself being positioned in dependence on the pressure in the third oil line S7.

In some such arrangements, as indicated in FIGURE 2, a number of different relief valves 120, 121, 122, having different settings to each other are incorporated in the third oil line 87 and are operated by solenoids 123, 124, to bring the appropriate relief valve into effective operation.

In other such arrangements each step in pressure in the third oil line S7 is controlled by a solenoid operating a relief valve which is disposed in the third oil line and which is capable of being gagged a desired number of times to provide the different pressures. For example, as indicated in FIGURE 3, the relief valve may comprise a pressure modulating device 126 of known type having a progressive action under the control of an electromagnetic device 127. In this case selection of stop withdrawal would depend on the extent of energiz'ation of the electromagnetic device 127. Fluid from the third oil line which passes through the relief valve or valves is led through the coarse pitch conduit 25 to drain.

The system described also refers to the provision of an increased tine pitch pressure arrangement for stop withdrawal. This feature does not form part of the present invention but is merelyV described in order to show how the invention may be applied vto a propeller system of the two-lock type as already disclosed in U.S. Patent Nos. 3,024,848 and 2,934,153. Thus the invention may be employed in systems relying only on a single independent line for stop withdrawal.

We claim:

1. A variable pitch propeller having pitch control means comprising a hydraulic piston-and-cylinder motor connected to the propeller blades to alter the pitch thereof, a plurality of stops operable to prevent relative movement of the piston and cylinder, in a sense to tine the propeller pitch, beyond respective predetermined line pitch settings, supply conduits for supplying pressure uid to opposite ends of the cylinder of said motor, a source of pressure uid, a further fluid conduit which is independent of said supply conduits and which is connected to said source, means for effecting successive increases in the pressure in said further conduit, and withdrawal means for Withdrawing said stops successively when said successive increases of pressure occur in said further fluid conduit.

2. A variable pitch propeller as claimed in claim 1, wherein the means for effecting successive increases in the pressure in said further condiut comprises a relief valve disposed in said further conduit and means selectively operable to adjust the pressure at which the relief valve operates.

3. A variable pitch propeller as claimed in claim 1, wherein the means for effecting successive increases in the pressure in said further conduit comprises a plurality of relief valves disposed in the conduit and set to operate at diierent pressures.

4. A variable pitch propeller having pitch control means comprising a hydraulic piston-and-cylinder motor connected to the propeller blades to alter-the pitch thereof, a plurality of stops operable to prevent relative movement of the piston and cylinder, in a sense to ne the propeller pitch, beyond respective predetermined ne pitch settings, supply conduits for supplying pressure uid to opposite ends of the cylinder of said motor, a source of pressure fluid, a further iluid conduit which is independent of said supply conduits and which is connected to said source, means for effecting successive increases in the pressure in said further conduit, an auxiliary piston-andcylinder motor whereof the piston is engageable with the stops to Withdraw them successively with said successive increases in pressure in the further conduit, a springloaded valve in said further conduit which valve provides a plurality of spaced ports and a valve member which is movable against the spring loading by different amounts in response to successive increases of pressure in the further conduit to open said ports in succession, said ports serving when opened by the valve member to permit uid from the further conduit to flow therethrough to act on the piston of the auxiliary motor in a sense to Withdraw the stops, and follow-up means connected to move with the piston of the auxiliary motor and operative to close 01T the ports opened by the valve member.

5. A variable pitch propeller having pitch control means comprising a hydraulic piston-and-cylinder motor connected to the propeller blades to alter the pitch thereof, two supply conduits for supplying pressure fluid to respective ends of the cylinder of said motor, a source of pressure uid, a third uid conduit which is independent of said tWo supply conduits and which is connected to said source, at least two stops operable to prevent relative movement of the piston and cylinder, in a sense to fine the propeller pitch, beyond respective predetermined ne pitch settings, selectively operable means for effecting successive increases in the pressure in said third conduit, an auxiliary piston-and-cylinder motor whereof the piston is enagageable with the stops to Withdraw them successively with said successive increases in pressure in the third conduit, a spring-loaded valve in said third conduit, which valve provides port means for the respective stops and a valve member controlling the port means, the port means for each stop being spaced from that for each other stop, the valve member being movable against the spring loading by different amounts in response to said successive increases of pressure in the third conduit to open said port means in succession, and each port means serving when opened by the valve member to permit pressure fluid from the said third conduit to flow therethrough to act on the piston of the auxiliary motor to withdraw the stop associated with that port means, and follow-up means connected to move with the piston of the auxiliary motor and operative to close olf the port means opened by the valve member as the movement of the piston of the auxiliary motor withdraws the associated stop.

6. A variable pitch propeller as claimed in claim 2, comprising solenoid means operatively connected to the relief valve.

References Cited in the le of this patent UNITED STATES PATENTS 2,655,999 Basevi Oct. 20, 1953 2,934,154 Chilman Apr. 26, 1960 2,944,769 Godden et al July 12, 1960 3,024,848 Chilman et al. Mar. 13, 1962 FOREIGN PATENTS 1,185,252 France Feb. 9, 1959 818,612 Great Britain Aug. 19, 1959 

1. A VARIABLE PITCH PROPELLER HAVING PITCH CONTROL MEANS COMPRISING A HYDRAULIC PISTON-AND-CYLINDER MOTOR CONNECTED TO THE PROPELLER BLADES TO ALTER THE PITCH THEREOF, A PLURALITY OF STOPS OPERABLE TO PREVENT RELATIVE MOVEMENT OF THE PISTON AND CYLINDER, IN A SENSE TO FINE THE PROPELLER PITCH, BEYOND RESPECTIVE PREDETERMINED FINE PITCH SETTINGS, SUPPLY CONDUITS FOR SUPPLYING PRESSURE FLUID TO OPPOSITE ENDS OF THE CYLINDER OF SAID MOTOR, A SOURCE OF PRESSURE FLUID, A FURTHER FLUID CONDUIT WHICH IS INDEPENDENT OF SAID SUPPLY CONDUITS AND WHICH IS CONNECTED TO SAID SOURCE, MEANS FOR EFFECTING SUCCESSIVE INCREASES IN THE PRESSURE IN SAID FURTHER CONDUIT, AND WITHDRAWAL MEANS FOR WITHDRAWING SAID STOPS SUCCESSIVELY WHEN SAID SUCCESSIVE INCREASES OF PRESSURE OCCUR IN SAID FURTHER FLUID CONDUIT. 